Functions

See https://kotlinlang.org/docs/functions.html

typealias:

// https://kotlinlang.org/docs/type-aliases.html
typealias ClickHandler = (Button, ClickEvent) -> Unit

kotlinc -include-runtime -d hello.jar hello.kt

./code/functions/hello.kt

fun sum1(x: Int, y: Int): Int {
  return x + y
}

// ignore the {} and return
fun sum2(x: Int, y: Int): Int  = x + y

// ignore the return type hint
fun sum3(x: Int, y: Int) = x + y

fun testSum() {
  check(sum1(1, 2) == 3)
  check(sum2(1, 2) == 3)
  check(sum3(1, 2) == 3)
}

fun double(x: Double): Double {
  return x * 2
}

// we can use trailing comma in the function parameters
//
@OptIn(kotlin.ExperimentalStdlibApi::class)
fun powerOf(
  x: Int,
  exponent: Int,
  ): Int {
    var n = 1
    for (i in 0 ..< exponent) {
      n *= x
    }
    return n
}

fun default(a: Int, b: Int = 2): Int {
  return a + b
}

// we can call a.size in setting the default value for b!!!
fun default2(a: Array<Float>, b: Int = a.size): Float {
  return a[0] + b
}

fun testDouble() {
  val x = double(2.5)
  check(x == 5.0)
}

fun testPowerOf() {
  val x = powerOf(2, 3)
  check(x == 8)
}

fun testDefault() {
  val x = default(1)
  check(x == 3)

  val y = default(1, 3)
  check(y == 4)
}

fun testDefault2() {
  val a: Array<Float> = arrayOf(1.5f, 2.0f, 3.0f)
  val b = default2(a)
  check(b == 1.5f+3)

  val c = default2(a, 10)
  check(c == 1.5f+10)
}

fun testLambda() {
  val makeUppercase = {x: String -> x.uppercase()}
  check(makeUppercase("ab") == "AB")
}

fun testLambda2(a: Int, b: Int, f: (Int, Int)->Int): Int {
  return f(a, b)
}

fun testLambda3(a: Int, f: (Int)->Int): Int {
  return f(a);
}


fun main() {
  testSum()
  testDouble()
  testPowerOf()
  testDefault()
  testDefault2()
  testLambda()
  var i = testLambda2(2, 3) {a, b -> a + b}
  check(i == 5)

  i = testLambda2(2, 3) {a, b -> a - b}
  check(i == -1)

    // the default argument is named as it
  i = testLambda3(2) {it + 1}
  check(i == 2 + 1)
}